Acoustic guitar subtleties re-created by DSP

PORTLAND, Ore.  Guitarists usually associate digital signal processing with electric, rather than acoustic, guitars, especially after early attempts to apply DSPs to acoustic instruments led to modeling effects that kept the guitars from performing as advertised.

Now Fishman Transducers Inc. says it has repurposed the Analog Devices Inc. Blackfin DSP to perform "digital acoustic imaging" instead of modeling, making acoustic guitars with an inexpensive piezoelectric pickup sound as if they were in a pristine studio in front of an expensive condenser microphone.

"When we first considered digital signal processing for guitars, we were just thinking modeling--making a Gibson sound like a Fender," said Larry Fishman, president of Fishman Transducers (Wilmington, Mass.). "We tried that, but found that it's much too complicated a problem, with all the subtle complexities. Now what we do instead is make an acoustic guitar sound as good in your home recordings, or live onstage, as it does in a professional studio."

The idea, he said, "is to capture that great studio sound and bring it out to real world of performances."

Fishman's digital acoustic imaging algorithm works by comparing the sound of a guitar under perfect conditions—in an ultraquiet studio with a variety of expensive condenser microphones placed at various distances in front of it—with the signal you get from a piezoelectric transducer or pickup, which Fishman places under the bridge saddle. The transducer senses the originating excitation of the strings, but is not sensitive to the sound hole resonances. And it doesn't hear the mix of phases in front of the instrument as various frequencies radiate differently off the top, sides and head of the guitar.

Fishman says it can capture and re-create all these subtle frequency and phase differences by running its algorithm on a Blackfin DSP from Analog Devices (Norwood, Mass.), contouring the audio from the raw transducer so that it sounds as if were in the studio.

"First, we take a guitar into the studio and do a two-channel recording—one channel records from what the transducer hears and the other channel records what the microphone hears," said Fishman. "Then we run the algorithm we developed to make a very close comparison of the two signals in the frequency domain, subtract one from the other to get their difference, then convolve the two to get what we call a sound image."

From the sound image, Fishman Transducers creates a custom filter with more than 2,000 frequency taps for the Blackfin DSP. "We not only adjust the amplitude of each frequency, but also make critical-phase adjustments, which is where the magic comes in," Fishman said. "Without that phase information, we would just have a 2,000-band graphic equalizer. But by adjusting the phase information too, we get three-dimensionality in the sound."